<!DOCTYPE html>
<html lang="ch">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>


    <style>

        .box{
            width: 200px;
            height: 200px;
            background-color: yellow;
            position: relative;
            /* z-index: 100; */
        }

        .child{
            width: 100px;
            height: 100px;
            background-color: aqua;
            position: absolute;
            top: 20px;
            left: 20px;
            z-index: -1;
        }
/*  */
    </style>

</head>
<body>
    
<div class="box">

    <div class="child"></div>

</div>
</body>
</html>